1. Field of the Invention
The present invention relates to a substrate and a manufacturing method thereof, which is used especially as a so-called super twisted nematic type (STN) color liquid crystal display.
2. Description of Prior Art
In a basic structure of a color liquid crystal display, a color filter layer is formed on the surface of a glass substrate, on which a transparent electrode is provided. However, since sodium (Na) ions in the glass substrate and/or other ions in the color filter elute into the liquid crystal and shorten the lifetime of such a structure, there has been proposed a construction in which an ion barrier layer is provided between the color filter and the transparent layer (Japanese Laid Open Patent No. 61-233720 (1986)).
The color filter is an assembly of elements including red (R), green (G) and blue (B) filters (in some examples, black stripe (BL) elements are further added thereto). In the manufacturing method thereof, for example, an exposure process is performed on the surface of a black color light-sensitive resin through a mask, and a line-shaped black stripe is formed by removing the non-exposed portion with a developer such as alcohol. Further, the dot-shaped red color elements are formed by developing a red color light-sensitive resin, and green color elements and blue color elements are formed in the same manner.
Between each of the elements formed in such manner, especially between the black stripe elements (BL) and the others (R, G, B), there appears a step-like difference of a few hundred nm as shown in FIG. 9.
In the case where a transparent conductive film, such as an ITO(indium tin oxide) and so on, is formed on the uneven surface of such a color filter directly and further where a patterning treatment is performed on it, poor patterning and/or circuit breakage occur due to an undesirable influence of the unevenness.
Therefore, in the above document (Japanese Laid Open Patent No. 61-233720) and others, a construction has been proposed in which an overcoat layer of a resin material is formed on each of the elements of the color filter so as to form a transparent conductive layer thereon.
Since the overcoat layer is provided for achieving flatness on the surface of the color filter as mentioned above, if it is too thin, the overcoat layer cannot accomplish the intended purpose thereof.
However, if it is formed in a thickness sufficient to achieve good surface evenness, a gas is generated when the transparent conductive layer is formed on the overcoat layer. Due to the gas, the productive efficiency is greatly decreased, the conductivity of the transparent conductive layer is deteriorated, and poor patterning results due to the change in etching characteristics. Further, even after production is completed, when the color liquid crystal is used under very heavy dynamic conditions, such as in a monitor for a television or in a navigation apparatus for an automobile, the gas generated from the overcoat layer (or from the color filter or the black matrix) attacks the transparent conductive film and deteriorates the durability thereof.
Therefore, as disclosed in Japanese Patent Publication No. 5-37453 (1993), Japanese Patent Publication No. 7-703 (1995), Japanese Patent Publication No. 7-62086 (1995) and Japanese Patent Publication No. 7-62087 (1995), by applying a coating of silicon dioxide to the color liquid crystal, it is possible to form a layer of an inorganic oxide such as silicon dioxide, or a layer of a nitride, on the color filter or on the overcoat layer so as to prevent the generation of gas therefrom.
In the disclosure of Japanese Patent Publication No. 5-37453 (1993), Japanese Patent Publication No. 7-703 (1995), Japanese Patent Publication No. 7-62086 (1995) and Japanese Patent Publication No. 7-62087 (1995), the thickness of the sub-layer of the silicon dioxide is 100 nm, but this is not enough to obtain the required flatness or evenness thereby.
On the contrary thereto, if making the layer thickness of the sub-layer thick enough for obtaining the required flattening, it is impossible to prevent the gas from being generated.